The restoration of the dental organ by an indirect adhesive restoration requires the conditioning of two surfaces: dental and prosthetic.
If the surface treatment of the dental organ is generally well controlled, because it often consists of the application of an adhesive system, the conditioning of the prosthetic surface can pose more problems due to the diversity of the protocols according to the nature materials. Let’s review these surface treatments.
Assembly of ceramics with a glass matrix (feldspathic and synthetic based on leucite or lithium disilicate) (fig. 1)
Stages of the conditioning of the prosthetic part when using a vitreous ceramic.
they are generally used to make restorations based on non-retentive or low-retentive preparations. The assembly mode alone will therefore ensure the retention of the restoration;
they are fragile. Gluing them will dissipate the stresses within them and thus give them mechanical strength.
Among the 3 families of glue available (bonding composite, complex glue with adhesive potential, self-adhesive glue), it is necessary to choose the bonding composites to assemble these ceramics. Their use requires prior treatment of the intrados of the prosthetic piece and conditioning of the dental tissues according to the protocol detailed below.
Conditioning of the intrados of the prosthetic part [1]
The acid creates crevices within the vitreous matrix, increasing the surface area developed for bonding and creating microretentions, allowing micromechanical locking of the bonding composite [2, 3].
The application time and the concentration of the acid depend on the nature of the ceramic [4]:
glass-ceramic reinforced with lithium disilicate: 5% acid treatment for 20 seconds. Note that for Celtra® Duo (Dentsply Sirona), the application time is 30 seconds with a 5% acid.
This step, carried out by the practitioner after fitting the prosthetic part, is sometimes carried out by the prosthetist. In this case, after the try-in step, the restoration having been contaminated by saliva, the practitioner will simply reactivate the surface by applying 35-37% phosphoric acid for 15 seconds [5].
Hydrofluoric acid rinsing and drying
Abundant rinsing with water for 60 seconds is essential to eliminate all traces of acid [4]. Even after rinsing with plenty of water, many crystals may remain on the etched surface, which may reduce adhesion. Magne and Belser therefore go further by offering rinsing and neutralization of the acid by placing the prosthetic part in an ultrasonic tank with baking soda for 60 seconds or in an ultrasonic tank with 95° alcohol for 5 minutes [6].
Application of a silane
After the prosthetic part has dried, a layer of silane is applied for 1 minute to the etched ceramic, then air-dried. This coupling agent allows the retention of the ceramic to the bonding composite thanks to its high wettability and its chemical contribution to adhesion. Indeed, silane is a bi-functional molecule which provides chemical bonding to silica [7], increasing the shear strength of ceramic/composite bonding [8].
Applying the adhesive
Once the silane has completely dried, the adhesive is applied to the ceramic. The adhesive system selected must be compatible with the bonding composite. The adhesive is not light cured at this stage. The prosthetic piece is therefore placed immediately in an inactinic chamber to avoid premature polymerization of the adhesive. Studies have shown that the application of adhesive on etched and silanated ceramic may not be necessary [9, 10]. This step can be considered as optional [10].
Tooth surface conditioning
Bonding to dental tissues should be done with a clean surgical field, free of saliva, blood and oral contaminants. The rubber dam, although not compulsory, remains, in our opinion, the most reliable means of having an optimal operating field.
The tooth is cleaned before the clinical trial of the prosthesis. Trial gels or temporary cement should be removed using a brush/cup and pumice.
the dentin has been sealed (hybridized) during tooth preparation: the tooth surface is then sandblasted with 30 µm alumina particles before applying 37% phosphoric acid for 15 seconds, before rinsing and drying;
the dentin has not been sealed (hybridized) during the dental preparation: the practitioner applies all the steps of the adhesive system selected on the dental preparation. Remember that there are adhesive systems with phosphoric acid etching (M&R3 and M&R 2), self-etching systems (SAM2 and SAM1) and universal adhesives used in an M&R or SAM mode. If a pre-etch adhesive is selected, the enamel is etched for 30 seconds and the dentin for 15 seconds. Whichever system is chosen, the practitioner will scrupulously follow the manufacturer’s protocol and recommendations.
Collage
When the prosthetic and dental surfaces are ready, the bonding composite is applied to the underside of the prosthetic piece, which is then positioned on the tooth to be restored. Excess glue is removed. The entire joint of the restoration is covered with glycerin so that the polymerization of the bonding composite takes place away from the oxygen in the air. The hardening of the bonding composite can then take place.
Conclusion
Among the factors influencing the performance of the bonded joint, the cleanliness of the surfaces and the respect of the surface treatments play a dominating role. Knowledge of the materials to be bonded and their surface conditioning are therefore essential to improve the durability of adhesive restorations.